1. Field of the Invention
The invention relates to a method of reutilizing high-boiling compounds within an integrated chlorosilane plant.
2. Description of the Related Art
In the various subprocesses of the production of polycrystalline silicon, various chlorosilane compounds including high-boiling dichlorosilanes and oligochlorosilanes (HBs) are formed. The expression “high-boiling compound, high-boiling dichlorosilane and oligochlorosilane” or “high boiler” refers here to a compound which consists of silicon, chlorine, and possibly hydrogen, oxygen and carbon, and has a boiling point higher than that of tetrachlorosilane (57° C./at 1013 hPa). The compounds are preferably disilanes HnCl6-nSi2 (n=0−4) and higher oligo(chloro)silanes which preferably have from 2 to 4 Si atoms and also disiloxanes HnCl6-nSi2O (n=0−4) and higher siloxanes which preferably have from 2 to 4 Si atoms, including cyclic oligosiloxanes and their methyl derivatives. In the following, low-boiling silanes having a boiling point of <40° C. under atmospheric conditions (1013 hPa) will be referred to as LBs for short.
Both the synthesis of trichlorosilane (TCS) from metallurgical silicon and HCl and the deposition of polycrystalline silicon (Poly) from trichlorosilane are based on thermal equilibrium processes of chlorosilanes, as are described, for example, in E. Sirtl, K. Reuschel, Z. ANORG. ALLG CHEM. 332, 113 216, 1964, or L. P. Hunt, E. Sirtl, J. ELECTROCHEM. SOC. 119(12), 1741-1745,1972. Accordingly, the trichlorosilane synthesis forms not only trichlorosilane and silicon tetrachloride (STC) but also dichlorosilanes and monochlorosilanes and also HBs according to a thermal equilibrium. The crude trichlorosilane from the trichlorosilane synthesis contains 0.05-5% of these HBs. In addition, about 20 ppm of various boron compounds, up to 200 ppm of TiCl4 and other metal chlorides such as FeCl2, FeCl3 and AlCl3 are formed in crude trichlorosilane production. These have to be separated from the products trichlorosilane and silicon tetrachloride.
Methods of separating trichlorosilane and silicon tetrachloride from the abovementioned HBs are known. Thus, U.S. Pat. No. 5,252,307, U.S. Pat. No. 5,080,804, U.S. Pat. No. 4,690,810 or U.S. Pat. No. 4,252,780 describe the concentration of the HB fractions contaminated with metal chlorides to 1% by weight to 50% by weight in the bottom offtake stream, followed by subsequent hydrolysis and disposal as hydrolysis residue. These processes result in silicon and chlorine losses and also in problems in disposing of the hydrolysate and the HCl-containing wastewater obtained [M. G. Kroupa in Proceedings. from SILICON FOR THE CHEMICAL INDUSTRY VI, pp. 201-207, Loen, Norway, Jun. 17-21, 2002].
Further undesirable high-boiling chlorodisiloxane fractions arise in the distillation and partial hydrolytic purification of chlorosilanes. These high-boiling fractions have hitherto likewise been disposed of as hydrolysis residues and HCl-containing wastewater, as described, for example, in U.S. Pat. No. 6,344,578 B1, U.S. Pat. No. 3,540,861 or U.S. Pat. No. 4,374,110.
Furthermore, it has been both theoretically deduced [E. Sirtl, K. Reuschel, Z. ANORG. ALLG CHEM. 332, 113 216, 1964; E. Sirtl et al., J. ELECTROCHEM. SOC. 121, 919-,1974; V. F. Kochubei et al., KINET. KATAL., 19(4), 1084, 1978] and demonstrated analytically [V. S. Ban et al., J. ELECTROCHEM. SOC. 122, 1382-, 1975] that HBs (hexachlorodisilane, pentachlorodisilane, tetrachlorodisilane and trichlorodisilane) are also formed in the deposition of polycrystalline silicon from trichlorosilane. These HBs, which are highly pure in respect of dopants and metals, are present in the bottom offtake stream from the polycondensate distillation, which can be converted by means of silicon tetrachloride at 600-1200° C. [WO02/100776 A1].
HBs can also be cracked in the presence of hydrogen in a low-temperature conversion in a fluidized-bed reactor [JPHei1-188414-Osaka Titanium 1988].
Polychlorosilanes (SinCl2n+2; 4≧n≧2), in particular Si2Cl6 (HCDS) decompose at z 700° C. in the presence of silicon crystal nuclei or on a heated silicon core [EP282037-Mitsubishi 1988]. It is also known that highly pure HCDS can be isolated from the offgases from the deposition of polycrystalline silicon [WO2002012122-Mitsubishi, 2002]. The cleavage of polychlorodisilanes by means of HCl over activated carbon can proceed even in the range from 30 to 150° C. [JP09-263405-Tokuyama 1996]. The reaction of this HB fraction together with silicon tetrachloride and hydrogen can be carried out in a high-temperature reactor (Dow Corning 2001 [US2002/0187096]). Disilanes from the direct synthesis of organosilanes can likewise be converted into trichlorosilane and/or silicon tetrachloride at 300° C. [U.S. Pat. No. 6,344,578 B1 Wacker 2000]. Low-temperature cleavage occurs in the presence of nucleophilic catalysts [F. Hoefler et al., Z. ANORG. ALLG. CHEM. 428, 75-82, 1977; DE3503262-Wacker 1985; G. Laroze et al, Proceedings, from SILICON FOR THE CHEMICAL INDUSTRY III, pp. 297-307, Trondheim, Norway, 1996; W.-W. du Mont et al, ORGANOSILICON CHEMISTRY V, Sep. 2001, Chem. Abst., 142:1555991; G. Roewer et al., SILICON CARBIDE—A SURVEY IN STRUCTURE AND BONDING 101, pp. 69-71, Springer 2002]. Lewis acids such as AlCl3 can likewise catalyze the cleavage of Si—Si bonds [A. Gupper et al., EUR. J. INORG. CHEM, 8, 2007-2011, 2001].
All these methods of removing undesirable HBs from processes for producing polycrystalline silicon involve a high engineering outlay for the disposal, separation and purification steps. In addition, losses of chlorine and silicon cannot be avoided.
The thermal decomposition of HBs in the presence of silicon tetrachloride and hydrogen is known from JPHei1-188414 of Osaka Titanium.